1. The Importance of Groundwater in Sustaining Streamflow in the Upper Colorado River Basin Matthew Miller Susan Buto, David Susong, Christine Rumsey, John Solder U.S. Geological Survey, Utah and Nevada Water Science Centers May 5 th, 2016

2. USGS National Water Census: Colorado River Basin Focus Area Study 1.Evapotranspiration 2.Snowpack Hydrodynamics 3.Water Use Information 4.Groundwater Discharge to Streams Summarized in Bruce and others (2015) – USGS Fact Sheet 2015-3080

3. Project Development and Coordination Objectives developed from a USGS meeting with selected stakeholders including Reclamation Strategy focused on filling in information gaps Lack of groundwater information in UCRB Groundwater and surface water are an interconnected resource Wealth of historic USGS data

4. BOR Colorado River Basin Supply and Demand Study Projected demand is greater than projected supply Median imbalance of ~ 3.2 million acre-feet by 2060 US Bureau of Reclamation (2012) Overarching Objective Provide quantitative information that adds new understanding of the joint groundwater- surface water resource in the UCRB

5. Study Objectives 1.Determine the spatial distribution of groundwater discharge to streams (where) 2.Quantify the fraction of total streamflow that is supported by groundwater discharge (how much) 3.Quantify the age of groundwater in the UCRB (vulnerability) USGS

6. Information from streamflow and chemical composition used to separate streamflow into runoff and groundwater components Chemical Hydrograph Separation Miller and others (2014, Water Resources Research)

7. USGS Long Term Records Long-term discharge and specific conductance data collected by USGS were used for hydrograph separation. Data from 1984-2012 used for present study 1947 2010

8. Groundwater Discharge to Streams Miller and others (2014, Water Resources Research) Hydrograph separation results for Colorado River at Cisco, UT Summary of 12 Stations in UCRB 1.“Pulse” of groundwater during snowmelt 2.Percent of total flow that is groundwater a.Annual: 21 – 58% b.Snowmelt: 13 – 45% c.Low-flow: 40 – 86%

9. Discrete data Continuous Data Problem: Given a small number of observed SC concentration values, and a continuous record of discharge how can we “fill in the gaps” in the SC record? Discharge Time Season

10. Groundwater Discharge to Streams – Estimation with Discrete Data Excellent fit between groundwater discharge to streams estimated using measured data and a regression approach at sites not directly impacted by anthropogenic activities Groundwater discharge to streams estimated at 229 sites using regression approach (sites filtered to remove those directly impacted by anthropogenic activities) Rumsey et al. (2015, J. Hydrol. – Reg. Stud.) Miller et al. (2015, J. Hydrol.)

11. Developed to estimate long-term mean steady state water quality conditions USGS SPARROW Model

12. Most groundwater discharge to streams occurs in upper elevation catchments Climate change in high elevation systems will affect the groundwater resource, and therefore the amount of surface water in streams 14.5 maf/yr = Groundwater discharge to streams in Upper Basin 2.7maf/yr = Water delivered to the Lower Basin that originated as groundwater Water lost during in-stream transport due largely to irrigation withdrawals and evapotranspiration Spatial Distribution of Groundwater Discharge Miller and others (in press, Water Resources Research)

13. Groundwater and Surface Water – A Single Resource Mill Creek Near Moab, UT (79% GWD)Colorado River Near Cisco, UT (55% GWD) Yampa River at Steamboat Springs, CO (34% GWD)

14. On average 56% of streamflow in the UCRB is estimated to be from groundwater discharge Streamflow in lower elevation catchments is more dependent on groundwater discharge than high elevation systems These streams support ecologically sensitive habitats (e.g. riparian zones) Streamflow Sustained by Groundwater Miller and others (in press, Water Resources Research)

15. Age of Groundwater in UCRB 20% of groundwater < 10 years old 65% < 100 years old Relatively young age of groundwater suggests potential rapid response to changes in environmental conditions Age Tracers – 3 H, Noble gases, CFCs, SF 6, 14 C

16. Summary and Implications 1.Any processes that affect the groundwater resource will also affect the surface water resource – Now have quantitative estimates of groundwater discharge to streams 2.Most groundwater discharge to streams occurs in upper elevation watersheds 3.Streamflow in lower elevation systems is dependent on groundwater discharge to streams 4.Most groundwater is young, suggesting the potential for rapid response of groundwater discharge to environmental change 5.Collaborating with Reclamation on study that will use this model as a tool to predict future groundwater discharge to streams under different climate scenarios

17. Matt Miller USGS, Utah Water Science Center mamiller@usgs.gov Thank you